There are many liquid crystal display devices each of which contains a backlight unit to backlight a panel so as to realize a bright display screen.
In recent years, since mobile devices, such as mobile phones are becoming thinner, liquid crystal display devices included in the mobile devices are also required to be thinner. In order to meet this demand, there has been proposed an edge light type (side light type) backlight unit.
The following description discusses a conventional edge light type backlight unit with reference to FIG. 6. FIG. 6 is a cross-sectional view showing a liquid crystal display device 200 which includes a backlight unit 210. The backlight unit 210 includes (i) a light guide plate 202 that is provided so as to face a liquid crystal panel 220, (ii) a light source 201, such as an LED, which is provided along a side of the light guide plate 202, and (iii) a frame 203 which holds the light guide plate 202 and the light source 201. Light emitted from the light source 201 and entered the light guide plate 202 propagates through the light guide plate 202, and exits through the top surface of the light guide plate 202. The light illuminates the liquid crystal panel 220.
Note here that, according to the backlight unit 210 shown in FIG. 6, the light that has entered the light guide plate 202 suffers from light energy loss. For example, (i) light that has reached an end surface 202a, which is opposite a light incident surface of the light guide plate 202, is scattered due to roughness of the end surface 202a and/or (ii) light that leaked from the end surface 202a is scattered by the frame 203. As a result, light from the light source 201 is not fully utilized. Moreover, if such scattered light enters the liquid crystal panel 230, the light is displayed as a bright line. This causes another problem of reducing display quality of the liquid crystal display device 200.
The Patent Literature 1 discloses a technique to reduce light that is scattered in a light guide plate. FIG. 7 shows a backlight unit 310 described in Patent Literature 1. As shown in FIG. 7, according to the backlight unit 310, a reflective diffraction element 303 is provided on an end surface 302a of a light guide plate 302, which end surface 302a is on the side opposite to a light source 301. This causes light that has conventionally escaped from the end surface 302a to be diffracted by the reflective diffraction element 303, and to be outputted through the top surface of the light guide plate 302. This improves light use efficiency of the backlight unit 310.